The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Thixotropic materials have the property of being fluid under shear but developing a gel structure and becoming self-supporting when at rest. The process is reversible. Thixotropic cements, which are thin and fluid during mixing and placement but reversibly form a rigid gel structure when pumping ceases, are useful in various subterranean applications, including water wells and hydrocarbon producing wells such as gas or oil wells. Some useful applications of thixotropic cement systems are plugging lost circulation zones during both drilling and cementing operations, repair of damaged or corroded casing, as grouts, and to limit annular gas migration in some situations. Such systems have also been used in situations where weak formations are exposed and would otherwise fracture under the hydrostatic pressure of a cement column; when using a thixotropic cement, the hydrostatic pressure of the column diminishes as the cement gels.
Incorporation of LAPONITE (REGISTERED TRADE MARK—synthetic smectite clay products for water-based applications) into a cement slurry yields a highly thixotropic cement, as described in U.S. Pat. No. 6,279,655, owned by the assignee to this application, and incorporated herein by reference in its entirety. When thixotropic cement is mixed on the surface prior to subterranean use, a highly viscous cement slurry is formed. Pumping such a viscous thixotropic cement slurry from the surface to the target depth is traditionally achieved by the use of pressure pumps to transport the slurry through coiled tubing or through placement conduit, e.g., drillpipe, casing, and the like.
Some issues that can be associated with the conventional pumping techniques for such cement slurries, include: (1) high pumping pressure which can exceed maximum working pressure of surface equipment; (2) loss of thixotropic properties caused by shear stress on the slurry as it is pumped from the surface to the target depth, (3) loss of thixotropy resulting in the cement containing LAPONITE falling back through the slots of a slotted liner or liner leak. The negative consequences associated with this include stuck pipe and poor cement/formation bond in the zone of interest. The phrase “stuck pipe”, as used herein, refers to the placement mechanism (e.g. coiled tubing) becoming lodged, or stuck, due to the differential pressure that is caused by the cement reentering the inner liner.
Thus, there is a continuing need for improved ways to deliver and use thixotropic cement slurries in hydrocarbon or water bearing subterranean formations. This invention meets at least some of that need.